Iron oxide nanoparticles have magnetic properties that depend on their size.
- superparamagnetic when they are smaller than about 20 nm. In this case, their magnetic moment is not strong enough to be thermally stable at physiological temperature;
- ferrimagnetics when they are of intermediate sizes, between about 20 and 80 nm. The magnetic moment of the nanoparticles is then thermally stable at physiological temperature;
- multidomain when their sizes exceed 80 nm. The individual magnetic moments present in the different domains cancel each other out, which leads to weakly magnetic nanoparticles.
The figure below summarizes the magnetic properties of magnetic nanoparticles according to their size.